The present invention relates to ends for can-type containers. More specifically, the invention pertains to a method and an apparatus for manufacturing a relatively thin can end with minimal warpage.
Can-type containers used for the storage of food products often comprise a body and two ends fixed to the body. Manufacturers of can ends, in general, make substantial efforts to reduce the thickness of the can ends which they produce. Reducing the thickness of a can end lowers the amount of material needed to manufacture the can end, and thereby leads to cost savings. For example, thickness reductions as small as one-thousandth of an inch or less can yield substantial cost savings over time due to the relatively large production volumes of typical can ends. Hence, the ability to manufacture can ends from relatively thin sheets of material offers substantial benefits. For example, the use of double-reduced steel in the manufacture of can ends is particularly advantageous because double-reduced steel provides a favorable combination of thinness, tensile strength, hardness, and resistance to elongation.
Reducing the thickness of a can end, however, increase the potential for the can end to warp during manufacture. Can ends manufactured from materials formed by rolling, e.g., double-reduced steels, are particularly susceptible to such warpage. In particular, the rolling operation induces a direction-dependent non-uniformity in the mechanical properties of the can end, i.e., rolling causes the mechanical properties of the can end to vary in different directions. This non-unifornity induces a tendency in the can end to warp. Warpage of a can end inhibits the effective mating of the can end and the can body. In addition, warpage can interfere with the automated transfer (feeding) of the can end during subsequent processing operations, e.g., lining of the can end. Hence, can-end warpage is highly undesirable and should be minimized or eliminated.
Warpage of a can end can be reduced by coining an annularly-shaped area on the can end. Coining substantially reduces the directional non-uniformity in the mechanical properties of the coined area, and thereby lowers or eliminates the tendency of the can end to warp. Coining, however, usually increases the diameter of the can end. In particular, the coining operation causes material within the coined area to be displaced. The displacement of material in this manner usually causes an increase in the chuck-wall diameter of the can end. Increases in chuck-wall diameter can inhibit the effective mating of the can end and the can body. Furthermore, increases in the chuck-wall diameter can prevent a proper fit between the can end and the seaming chuck utilized to join the can end to a can body. Hence, increases in chuck-wall diameter resulting from the coining operation should be minimized or eliminated.
The above-described increase in chuck-wall diameter is illustrated in FIGS. 13A and 13B. FIG. 13A shows a can end 100 having a chuck wall 102 and a panel 104. FIG. 13A depicts the can end 100 before the panel 104 is coined. The panel 104 has an initial length denoted by the symbol xe2x80x9cL1.xe2x80x9d The can end 100 has an initial chuck-wall diameter represented by the symbol xe2x80x9cD1.xe2x80x9d
FIG. 13B depicts the can end 100 after the panel 104 has been coined. The material displaced by the coining operation increases the overall length of the coined panel 104 by an amount represented by the symbol xe2x80x9cxcex941.xe2x80x9d Hence, the overall length of the panel 104 after the coining operation equals the initial length (L1) plus the increase in the length of the panel 104 caused by the coining operation (xcex941). The increase in the length of the panel 104 causes a corresponding increase in the chuck wall diameter of the can end 100. Specifically, the chuck-wall diameter after the coining operation is approximately equal to the initial chuck-wall diameter (D1) plus the change in the length of the panel 104 (xcex941).
The above discussion illustrates the current need for a method and an apparatus for manufacturing a relatively thin can end with minimal warpage. More particularly, a method and an apparatus are needed for reducing the tendency of thin can ends to warp during manufacture, without substantially affecting the chuck-wall diameter of the can ends. The present invention is directed to these and other goals.
An object of the present invention is to provide a method of forming a can end with minimal warpage. In accordance with this object, a presently-preferred method of forming a can end comprises the step of forming a substantially circular metal blank having a periphery and a center panel. The method also comprises the step of forming a substantially annular recessed panel in the blank. The recessed panel has a first depth in relation to a substantially annular portion of the blank contiguously formed with the recessed panel. The method further comprises the step of coining the substantially annular portion of the blank while re-forming the recessed panel to a second depth in relation to the substantially annular portion of the blank, with the second depth being greater than the first depth.
Further in accordance with the above-noted object, another presently-preferred method of forming a can end comprises the step of forming a substantially circular metal blank having a periphery and a center panel. The method also comprises the step of forming a substantially annular recessed panel in the blank. The recessed panel has a depth in relation to a substantially annular portion of the blank contiguously formed with the recessed panel. The method further includes the step of coining the substantially annular portion of the blank while the recessed panel is being formed, and after the depth of the recessed panel reaches a predetermined value.
Another object of the present invention is to provide a method for minimizing warpage of a can end. In accordance with this object, a presently-preferred method of minimizing warpage of a can end comprises the step of partially forming a substantially annular recess in the can end and then fully forming the recess while coining a substantially annular area of the can end bordering the recess.
A further object of the present invention is to provide an apparatus for forming a can end with minimal warpage. In accordance with this object, a presently-preferred embodiment of a die for forming a can end comprises an annular cut edge having an inner circumferential surface. The die also comprises a punch coaxially disposed with the cut edge. The punch and the cut edge are adapted to form a metal blank having a periphery and a center panel.
The die further comprises means for forming an annular recessed panel in the blank. The recessed panel has a first depth in relation to a substantially annular portion of the blank contiguously formed with the recessed panel. The die also comprises means for coining the substantially annular portion of the blank while re-forming the recessed panel to a second depth in relation to the substantially annular portion of the blank. The second depth is greater than the first depth.